Polyurethanes and epoxy resins are versatile materials that find use as elastomers, coatings, encapsulating agents and composite matrices. Curing compositions having amines, such as aliphatic or aromatic amines, are widely used to cure epoxy or isocyanato-terminated prepolymers to form such epoxy resins and polyurethanes. Conventional amine curing compositions are described in U.S. Pat. Nos. 3,876,604; 4,075,150; and 4,282,344, the entire contents and disclosures of which are hereby incorporated by reference.
One type of amine curing composition is derived from aromatic amine compounds. The aromatic amine curatives currently available provide good materials for many applications, but suffer from several problems. One problem associated with the common aromatic diamines, when used as a curing composition for either epoxy or isocyanato-terminated prepolymers, relates to their hygienic considerations in the workplace. Many single ring and double ring diamines, such as methylenedianiline (MDA), are toxic or carcinogenic. Thus, these amines are regulated and the amounts of aromatic amines must be controlled to limit the toxic effect.
Aliphatic amines have also been used in the past for curing epoxy or isocyanato-terminated prepolymers. While it is known that aliphatic amines provide accelerated reaction rates compared to aromatic amines, aliphatic amines also have problems. For example, cycloaliphatic curatives permit rapid low temperature curing but leave residual NH functionality in the cured product that reacts with ambient carbon dioxide to form surface carbamates. These carbamates lead to water spotting and poor interfacial adhesion and, as a result, the product may be unsuited for subsequent coating and composite applications. In addition, the rapid curing of cycloaliphatic polyamines results in incomplete incorporation of the curative into the final product resulting in degradation of surface and interfacial properties. Thus, while aliphatic amines provide low temperature curing and reduced toxicity as compared to aromatic amines, the above-identified problems limit the use of aliphatic amines.
Thus, the need exists for an effective curing composition that improves the properties of the epoxy or isocyanato-terminated prepolymers without the processing and physical problems of currently available aliphatic amines or the toxicity problems of aromatic amines.